Process for the preparation of aldehydes

ABSTRACT

A process for the preparation of an aldehyde by reacting the corresponding aryl methanol with a stable free radical nitroxide in the presence of a NO x  -generating compound at a temperature in the range of from about 0° C. to about 100° C. and thereafter separating out the aldehyde.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of aldehydes bythe oxidation of the corresponding aryl methanols in the presence of astable free radical nitroxide and a NO_(x) -generating compound.

BACKGROUND OF THE INVENTION

It is known to use nitroxyl radicals/oxoammonium salts in the oxidationof primary alcohols to produce aldehydes and acids and secondaryalcohols to ketones (Journal of Organic Chemistry, vol. 52 (12), pp.2559-2562, and Journal of Organic Chemistry, vol. 55, 1990, pp.462-466).

It is reported in the open literature that primary aliphatic alcoholscan be converted to aldehydes in 30-40% yields in the presence ofcatalytic amounts of cuprous chloride,2,2,6,6,-tetramethylpiperidine-1-oxyl, and atmospheric oxygen (Journalof American Chemical Society, 1984, 106, pp. 3374). It is also knownthat higher yields of aldehydes can be obtained if stoichiometricamounts of cupric or ferric salts are used instead of catalytic amountsof the cuprous salts (Pure and Applied Chemistry, vol. 62(2), 1990, pp.217-222).

It has been found that aldehydes can be produced in good yields and withhigh selectivities from aryl methanols without producing large amountsof other products such as acids and esters. This can be accomplished byusing catalytic amounts of a stable free radical nitroxide, a NO_(x)-generating compound and optionally, an oxidant and/or a solvent.

SUMMARY OF THE INVENTION

This invention relates to a process for the preparation of an aldehydewhich comprises reacting the corresponding aryl methanol with a stablefree radical nitroxide having the formula: ##STR1## wherein (1) (a) eachof R₁, R₂, R₃ and R₄ is an alkyl, aryl or substituted alkyl group having1 to about 15 carbon atoms, and (b) R₅ and R₆ (i) each is an alkyl grouphaving 1 to about 15 carbon atoms provided that R₁ -R₆, are not allalkyl groups, or a substituted alkyl group having 1 to about 15 carbonatoms wherein the substituent is hydrogen, cyano, --CONH₂, --OCOCH,OCOC₂ H₅, carbonyl, alkenyl wherein the double bond is not conjugatedwith the nitroxide moiety, or --COOR wherein R of the --COOR group isalkyl or aryl, or (ii) together form part of a ring that contains atleast 3 carbon atoms and up to two heteroatoms of O or N,

or

(2) the ##STR2## moiety and the ##STR3## moiety individually are aryl,or (3) the ##STR4## moiety and the ##STR5## moiety together form abicyclic ring with the proviso that the group directly adjacent to theN--O moiety is a bridgehead C--H, or a fully alkylated carbon, in thepresence of a NO_(x) -generating compound and optionally, a solventand/or an oxidant, at a temperature in the range of from about 0° C. toabout 100° C. and thereafter separating out the aldehyde.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process converts aryl methanol s to the correspondingaldehydes by contacting the aryl methanol with a stable free radicalnitroxide in the presence of a NO_(x) -generating compound, a solventand an oxidant at a temperature in the range of from about 0° C. toabout 100° C. and thereafter separating out the aldehyde.

The aryl methanol reactant suitably comprises one or more aryl methanols, i.e., Ar-- CH₂ OH, having a carbon number in the range of from about5 to about 50, preferably from about 6 to about 30, and more preferablyfrom about 6 to about 20. Aryl methanols typically suitable for use inthe present invention include benzyl alcohol, naphthyl alcohol,anthracenyl alcohol, furfuryl alcohol and the like. It is contemplatedthat these aryl methanols may have any number of substituents which donot interfere with the oxidation of the hydroxy group. Suitablesubstituents include sulfonyl, nitro, chloro, carbamoyl and the like.

The process of the instant invention is particularly suited to arylmethanols with about 6 to about 30, preferably about 6 to about 20carbon atoms. In a preferred embodiment, the aryl methanol reactant i sselected from the group consisting of benzyl alcohol, 4-nitrobenzylalcohol, 3-chlorobenzyl alcohol, 4-methylbenzyl alcohol, 4-chlorobenzylalcohol, 3-methylbenzyl alcohol, 3-ethylbenzyl alcohol, 3-methoxybenzylalcohol and mixtures thereof, with benzyl alcohol being particularlypreferred.

The term "stable free radical nitroxide" as used herein shall mean afree radical nitroxide that can be prepared by conventional chemicalmethods and will exist long enough to be used in a subsequent chemicalreaction or examined in a static system by normal methods ofspectroscopy. Generally, the stable free radical nitroxides of thepresent invention have a half life of at least one year. The term"stable free radical" shall also be understood to include the precursorto a stable free radical from which the stable free radical may beproduced in situ.

The stable free radical nitroxides, as used in the present process, areprecursors to catalysts, i.e., oxoammonium salts, active for theoxidation of aryl methanols to the corresponding aldehydes. Thesecatalysts are generated in situ by the oxidation of a stable freeradical nitroxide to an oxoammonium salt with an oxygen-containingoxidant. The stable free radical nitroxide can be obtained by theoxidation of amines or hydroxyl amines.

The stable free radical nitroxides which are suitable for use in theinstant invention have the formula: ##STR6## wherein each of R₁, R₂, R₃and R₄ is an alkyl, aryl or substituted alkyl groups and no hydrogen isbound to the remaining valences on the carbon atoms bound to thenitrogen. As used herein, the term "alkyl" is meant to includecycloalkyl. The alkyl (or substituted) groups R₁ -R₄ may be the same ordifferent, and preferably contain 1 to 15 carbon atoms. Preferably, R₁-R₄ are methyl, ethyl, or propyl groups. In addition to hydrogen, thesubstituents may include, halogen, oxygen, nitrogen and the like.

The remaining valences (R₅ and R₆) in formula III above may be satisfiedby any atom or group except hydrogen which can bond covalently tocarbon, al though some groups may reduce the stabilizing power of thenitroxide and are undesirable. When R₁, R₂, R₃ and R₄ are each alkylgroups, however, at least one of R₅ and R₆ must be an aryl group.Preferably, R₅ and R₆ are substituted alkyl groups having I to about 15carbon atoms wherein the substituent is selected from halogen, cyano,--COOR, wherein R is alkyl or aryl, --CONH₂, --OCOC₂ H₅, carbonyl, oralkenyl where the double bond is not conjugated with the nitroxidemoiety, or alkyl groups of 1 to about 15 carbon atoms.

The remaining valences (R₅ and R₆) in formula III above may also form aring containing at least three carbon atoms and up to two heteroatoms,such as O or N. R₅ and R₆ can, for example, form a five-membered ringcontaining 3 carbon atoms and up to two heteroatoms, such as O or N, afive-membered ring containing 4 carbon atoms, a six-membered ringcontaining 5 carbon atoms, a seven-membered ring containing 6 carbonatoms, an eight-membered ring containing 7 carbon atoms, etc. Forpurposes of this invention, it is preferred that R₅ and R₆ together forma five-membered ring, a six-membered ring, a seven-membered ring, or aneight-membered ring, although larger rings would also be suitable.Examples of suitable compounds having the structure above and in whichR₅ and R₆ form part of the ring are2,2,6,6,-tetramethylpiperidine-1-oxyl,2,2,5,5-tetramethylpyrrolidin-1-oxyl,2,2,7,7-tetramethylcycloheptan-1-oxyl, mixtures thereof, and the like.It is understood that these compounds may contain substituents which donot interfere with the reaction.

The ##STR7## moieties in formula III above can individually be aryl,i.e., ##STR8## Examples of suitable compounds having the structure abovein which the ##STR9## moieties are individually aryl arediphenylamine-n-oxyl, phenol tertiary butylamine-N-oxyl,3-methyl-diphenylamine-N-oxyl , 2-chlorophenylamine-N-oxyl and the like.These compounds may be substituted with an substituents which do notinterfere with the reaction.

The ##STR10## moieties in formula III above can also form a bicyclicring wherein the group adjacent to the N--O moiety is either abridgehead C-H or a quaternary carbon. As used herein, the term"bridgehead C--H" refers to a tertiary carbon which is common to bothrings of the bicyclic ring system. As used herein, "a quarternarycarbon" refers to a fully substituted carbon atom having alkyl, aryl orsubstituted alkyl groups having 1 to about 18 carbon atoms assubstituents. Examples of suitable compounds having the structure abovein which the ##STR11## moieties form a bicyclic ring are2-azabicyclo[2.2.1 ]heptan-2-oxyl,2-azabicyclo[2.2.2]-3,3-dimethyloctan-2-oxyl,3azabicyclo[3.2.2]-2,2,4,4-tetramethylnonan-3-oxyl and the like. Thesecompounds may be substituted with any substituents which do notinterfere with the reaction.

In a preferred embodiment, the stable free radical nitroxide is apiperidine-1-oxyl having the formula: ##STR12## wherein each of R₇, R₈,R₉ and R₁₀ is an alkyl, aryl or substituted alkyl group having 1 toabout 15 carbon atoms and no hydrogen is bound to the remaining valenceson the carbon atoms bound to the nitrogen, and each of R₁₁ and R₁₂ isalkyl, hydrogen, aryl or a substituted heteroatom. As used herein, theterm "alkyl" is meant to include cycloalkyl. The alkyl (or substituted)groups R₇ -R₁₀ may be the same or different, and preferably contain 1 to15 carbon atoms. Preferably, R₇ -R₁₀ are methyl, ethyl, or propylgroups. In addition to hydrogen, the substituents may include, halogen,oxygen, nitrogen and the like. Typically, one of R₁₁ and R₁₂ ishydrogen, with the other one being a substituted heteroatom which doesnot interfere with the reaction. Suitable substituted heteroatomsinclude ##STR13## --O--polymer and the like.

In a particularly preferred embodiment, the nitroxide is selected fromthe group consisting of 2,2,6,6-tetramethylpiperidine-1-oxyl,4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl,4-ethoxy-2,2,6,6-tetramethylpiperidine-1-oxyl ,4-carbamoyl-2,2,6,6-tetramethyl-piperidine-1-oxyl,4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl,4-benzoylamino-2,2,6,6-tetramethylpiperidine- 1-oxyl 4-pivaloylamido-2 26 6-tetramethylpiperidine-1oxyl, 4-decyloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl,4-dodecanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl,4-octanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl and mixturesthereof, with2,2,6,6-tetramethylpiperidine-1-oxy),4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl,and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl being especiallypreferred.

The NO_(x) -generating compound in the present process is typicallyselected from the group consisting of an alkali metalnitrosodisulfonate, nitric acid and mixtures thereof, with nitric acidbeing preferred. However, any compound which serves to generate NO_(x)during the course of the reaction and which does not interfere with thereaction would be suitable. While not wishing to be bound by anyparticular theory, it is believed that nitrogen oxides (NO_(x)) aregenerated in the reaction and are the active species in the reaction.

The alkali metal nitrosodisulfonate suitable for use as a NO_(x)-generating compound can be any alkali metal nitrosodisulfonate althoughpotassium nitrosodisulfonate is preferred. As used herein, the term"alkali metal" is used as a descriptor of the elements Group IA of thePeriodic Table of the Elements (Li, Na, K, Rb, Cs, Fr). The alkali metalnitrosodisulfonate is typically dissolved in water prior to being addedto the reaction mixture although it can be added as a solid after all ofthe other reactants have been added.

As used herein, the term "nitric acid" refers to nitric acid, fumingnitric acid or nitrous acid generated by contacting a nitrate or nitritesalt such as, for example, an alkali metal salt, a tetraalkylammoniumsalt, an alkaline earth salt or a rare earth salt, with a strong acidsuch as, for example, a mineral acid. The nitric acid suitable for useas a NO_(X) -generating compound in the present invention typically hasa concentration in the range of from about 50 percent to about 100percent, preferably about 70 percent. Generally, an amount of nitricacid in the range of from about 5 mole percent to about 200 molepercent, basis the moles of starting aryl methanol is utilized. Thenitric acid is typically added to the reaction mixture after all of theother reactants have been added.

The reaction of the present invention can be carried out in the presenceor absence of a solvent. A solvent is needed in the reaction when thereactants are not in liquid form at the reaction temperature. When thereaction is carried out in the presence of a solvent, the solvent isgenerally one in which the aryl methanol is readily soluble. Solventswhich are most suitable are those which are inert in the reaction. Thesolvent may be added to the reaction mixture, or alternatively, thenitroxide may be dissolved in the solvent prior to addition of thenitroxide to the reaction medium. The solvent is typically selected fromthe group consisting of heptane, acetonitrile, dichloromethane, glyme,tertiary amyl alcohol, carbon tetrachloride, chlorobenzene and mixturesthereof. In a preferred embodiment, the solvent is selected fromdichloromethane, acetonitrile, glyme, heptane and mixtures thereof. Theamount of solvent utilized in the process is typically in the range offrom about 0.5:1 to about 100:1, preferably from about 2:1 to about10:1, basis the weight of the starting aryl methanol.

The process of the present invention may be carried out in the presenceor absence of an oxidant. In a preferred embodiment, the process iscarried out in the presence of an oxidant. The oxidants suitable for usein the instant invention are those compounds which are capable, in thepresence of a NO_(x) -generating compound, of oxidizing the stable freeradical nitroxide to the oxoammonium salt. Suitable oxidants includeoxygen-containing gases such as pure oxygen and oxygen in air. Whereaspure oxygen can is preferred to accomplish the desired conversion, theoxygen can also be diluted with an inert gas such as nitrogen, helium,argon, or other similar gas. For purposes of increasing the reactionrate, higher O₂ pressures such as, for example, up to about 2000 psigcan be utilized. In a preferred embodiment, pure oxygen is used as theoxidant and it is bubbled into the reaction solution. In anotherembodiment, the reaction solution can be exposed or left open to theatmosphere.

The amounts and concentrations of the reactants utilized in the processof the instant invention can vary within wide ranges. The amount ofstable free radical nitroxide is typically in the range of from about0.1 mole percent to about 200 mole percent, preferably from about 5 molepercent to about 20 mole percent, basis the number of moles startingaryl methanol. Generally, the amount of NO_(x) -generating compound usedis in the range of from about 5 mole percent to about 200 mole percent,basis the number of moles of aryl methanol.

The process of the present invention is typically conducted under mildconditions, with good results being obtained using a temperature in therange of from about 0° C. to about 100° C., preferably about 20° C. toabout 70° C., and most preferably, about 35° C. to about 50° C. Reactionpressures are not critical although higher pressures can result inincreased reaction rates. Pressures in the range of from aboutatmospheric pressure up to about 2000 psig can be employed with goodresults.

The process of the instant invention can be carried out either batchwiseor continuously, using a stirrer equipped reactor or other well knowncontacting technique to achieve adequate mixing. Preferred reactionconditions, e.g., temperature, pressure, flow rates, etc., vary somewhatdepending on the specific nitroxide utilized and on the concentration ofthe nitroxide.

The process of the instant invention can be carried out in a variety ofways. For example, 0.056 moles of aryl methanol, and 0.006 moles of thenitroxide, and solvent, if any, may be added to the reaction vessel,followed by the addition of 0.011 moles of 70 percent nitric acid andbubbling oxygen through the reaction mixture. Following the reaction,the product may be separated from the reaction mixture usingconventional procedures such as extraction using a suitable extractionsolvent such as, for example, ethyl acetate; evaporation wherein thesolvent is stripped from the reaction mixture by using heat or vacuum.The reaction product can be purified by a number of conventional meanssuch as high temperature water washing or distillation.

Depending upon process conditions and the nitroxide used, theselectivity to aldehyde obtained by this invention can be greater thanabout 99%. It should be understood that if the reaction of the presentinvention is allowed to continue after formation of the aldehydes,carboxylic acids rather than aldehydes may be produced. The productsproduced by the instant process can be used in a variety of detergentapplications. For example, these products can be used as intermediatesin the preparation of esters, imines, amines and acids.

The ranges and limitations provided in the instant specification andclaims are those which are believed to particularly point out anddistinctly claim the present invention. It is, however, understood thatother ranges and limitations which perform substantially the samefunction in the same or substantially the same manner to obtain the sameor substantially the same result are intended to be within the scope ofthe instant invention as defined by the instant specification andclaims.

The process of this invention will be further described by the followingembodiments which are provided for illustration and are not to beconstrued as limiting the invention.

ILLUSTRATIVE EMBODIMENTS Example 1

6 Grams of benzyl alcohol, 1.0 gram of2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acidand 25 milliliters of dichloromethane were charged to a 100 milliliterround bottomed flask. O₂ was bubbled through this mixture at a rate of35 milliliters/minute at ambient pressure. The reaction temperature washeld at 35° C. over a 2-hour period. The results are presented in TableI.

Example 2

6 Grams of benzyl alcohol, I gram of2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acidand 25 milliliters of dichloromethane were charged to a 100 milliliterround bottomed flask. This mixture was left open to the air at ambientpressure. The reaction was held at 35° C. over a 2hour period. Theresults are presented in Table I.

Example 3

6 Grams of benzyl alcohol, 1 gram of2,2,6,6-tetramethyl-piperidine-1-oxyl, 1 gram of 70 percent nitric acidand 25 milliliters of dichloromethane were charged to a 100 milliliterround bottomed flask. The reaction was exposed to the atmosphere andheld at 35° C. over a 5-hour period. This mixture was then left open tothe air overnight. The results are presented in Table I.

Example 4

6 Grams of benzyl alcohol, 1 gram of2,2,6,6-tetramethyl-piperidine-1-oxyl and 2 grams of 70 percent nitricacid were charged to a 100 milliliter round bottomed flask. Air wasexposed to this mixture at ambient pressure. The reaction temperaturewas held at 35° C. over a 2-hour period. The results are presented i nTable I.

Comparative Example A

Comparative Example A was carried out in a manner similar to Example 1except that no nitroxide was used. The results are presented in Table I.

Comparative Example B

Comparative Example B was carried out in a manner similar to Example 1except that no nitric acid was used. The results are presented in TableI.

As can be seen in Table I, nitroxide and nitric acid are necessary forthe oxidation of the aryl methanol to proceed.

                  TABLE I                                                         ______________________________________                                        Oxidation of Aryl Methanols to Aldehydes                                                            % Sel.                                                              % Conversion                                                                            Aldehydes                                               ______________________________________                                        Example 1     41          >99                                                 Example 2     28          >99                                                 Example 3     60          >99                                                 Example 4     78           92                                                 Comparative   <1            0                                                 Example A                                                                     Comparative    0            0                                                 Example B                                                                     ______________________________________                                    

What is claimed is:
 1. A process for the preparation of an aldehydewhich comprises reacting the corresponding aryl methanol with a stablefree radical nitroxide having the formula: ##STR14## wherein (1) (a)each of R₁, R₂, R₃ and R₄ is an alkyl, aryl or heteroatom substitutedalkyl group having 1 to about 15 carbon atoms, and (b) R₅ and R₆ (i)each is an alkyl group having 1 to about 15 carbon atoms provided thatR₁ -R₆ are not all alkyl groups, or a substituted alkyl group having 1to about 15 carbon atoms wherein the substituent is hydrogen, cyano,--CONH₂, --OCOCH, OCOC₂ H₅, carbonyl, alkenyl wherein the double bond isnot conjugated with the nitroxide moiety, or --COOR wherein R of the--COOR group is alkyl or aryl, or (ii) together form part of a ring thatcontains at least 3 carbon atoms and up to two heteroatoms of O or N,(2)the ##STR15## moiety individually are aryl, or (3) the ##STR16## moietytogether form a bicyclic ring with the proviso that the group directlyadjacent to the N--O is a bridgehead C--H, or a fully alkylated carbon,in the presence of a NO_(x) -generating compound, at, a temperature inthe range of from about 0° C. to about 100° C. and thereafter separatingout the aldehyde.
 2. The process of claim 1 wherein said aryl methanolhas a carbon number in the range from about 5 to about
 50. 3. Theprocess of claim 2 wherein said aryl methanol has a carbon number in therange from about, 6 to about
 30. 4. The process of claim 3 wherein thearyl methanol is selected from the group consisting of benzyl alcohol,4-nitrobenzyl alcohol, 3-chlorobenzyl alcohol, 4-methylbenzyl alcohol,4-chlorobenzyl alcohol, 3-methylbenzyl alcohol, 3-ethylbenzyl alcohol,3-methoxybenzyl alcohol and mixtures thereof.
 5. The process of claim 4wherein the aryl methanol is benzyl alcohol.
 6. The process of claim Iwherein the stable free radical nitroxide is selected from the groupconsisting of 2,2,6,6,-tetramethylpiperidine-1-oxyl,2,2,5,5-tetramethylpyrrolidin-1-oxyl,2,2,7,7-tetramethylcycloheptan-1-oxyl,and mixtures thereof.
 7. The process of claim I wherein the stable freeradical nitroxide has the formula: ##STR17## wherein each of R₇, R₈, R₉and R₁₀ is an alkyl, aryl or heteroatom substituted alkyl group having 1to about 15 carbon atoms and each of R₁₁ and R₁₂ is alkyl, hydrogen,aryl or a substituted heteroatom.
 8. The process of claim 7 wherein thestable free radical nitroxide is selected from the group consisting of2,2,6,6-tetramethylpiperidine-1-oxyl,4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl,4-ethoxy-2,2,6,6-tetramethylpiperidine-1-oxyl,4-acetylamino-2,2,6,6-tetramethyl-piperidine-1oxyl,4-carbamoyl-2,2,6,6-tetramethylpiperidine-1-oxyl,4-benzoylamino2,2,6,6-tetra-methylpiperidine-1-oxyl,4-pivaloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl,4-decyloylamido-2,2,6,6-tetramethylpiperidine-1-oxyl,4-dodecanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyl,4-octanoylamino-2,2,6,6-tetramethylpiperidine-1-oxyland mixtures thereof.
 9. The process of claim 8 wherein the stable freeradical nitroxide is selected from the group consisting of2,2,6,6-tetramethylpiperidine-1-oxyl,4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxyl,4-methoxy-2,2,6,6-tetramethyl-piperidine-1-oxyl and mixtures thereof.10. The process of claim 1 wherein said process is carried out in thepresence of a solvent.
 11. The process of claim 10 wherein said processthe solvent is selected from the group consisting of heptane,acetonitrile, dichloromethane, glyme, tertiary amyl alcohol, carbontetrachloride, chlorobenzene and mixtures thereof.
 12. The process ofclaim 1 wherein said NO_(x) -generating compound is selected from thegroup consisting of nitric acid, an alkali metal nitrosodisulfonate andmixtures thereof.
 13. The process of claim 12 wherein said NO_(x)-generating compound is nitric acid.
 14. The process of claim 13 whereinsaid nitric acid is selected from the group consisting of fuming nitricacid, nitrous acid generated by contacting an alkali metal nitrite withmineral acid, nitric acid generated by contacting an alkali metalnitrate with mineral acid, and mixtures thereof.
 15. The process ofclaim 13 wherein the amount of nitric acid is in the range of from about5 mole percent to about 200 mole percent, basis the moles of startingaryl methanol.
 16. The process of claim 12 wherein said NO_(x)-generating compound is an alkali metal nitrosodisulfonate.
 17. Theprocess of claim 16 wherein the amount of alkali metalnitrosodisulfonate is in the range of from about 5 mole percent to about200 mole percent, basis the moles of starting aryl methanol.
 18. Theprocess of claim 16 wherein said alkali metal nitrosodisulfonate ispotassium nitrosodisulfonate.
 19. The process of claim 1 wherein saidaryl methanol is contacted with said stable free radical nitroxide,followed by the addition thereto of said NO_(x) -generating compound.20. The process of claim 19 wherein the amount of stable free radicalnitroxide is in the range of from about 0.1 mole percent to about 200mole percent, basis the number of moles of aryl methanol.
 21. Theprocess of claim 19 wherein the amount of NO_(x) -generating compound isin the range of from about 5 mole percent to about 200 mole percent,basis the number of moles of aryl methanol.
 22. The process of claim 1wherein said process is carried out in the presence of an oxidant. 23.The process of claim 22 wherein said oxidant is an oxygen-containinggas.
 24. The process of claim 23 wherein said oxygen-containing gas isselected from the group consisting of pure oxygen and air.
 25. Theprocess of claim I wherein said process is carried out at a temperaturein the range of from about 20° C. to about 70° C. and at atmosphericpressure.